Knee pain at the age of 13 may be the result of the following pathologic changes. First, it may be caused by patellar instability. Due to the influence of genetic factors, patients have patella, femur, patella, the shape of the relative articular surface is abnormal, which produces patellar bony instability. In addition, if such patients do not exercise regularly, the lack of muscle strength in the thigh can exacerbate the instability. Such patients are prone to develop the disease in adolescence, and once it develops, the knee joint is prone to patellofemoral pain. Sometimes, in the process of violently twisting the body, it will lead to a transient dislocation of the patella, which in turn will lead to severe pain in the knee joint with a fall. Second, it may be caused by chondromalacia of the patella. Seen in the patient’s own weight is too heavy, or do excessive physical exercise on the knee joint. This leads to damage to the articular cartilage on the relevant joint surface of the patella, followed by chondromalacia of the patella, resulting in pain. Third, it may be growing pains. When the patient is 13 years old, the body may be in a period of rapid growth and development. This may cause a short-term calcium deficiency in the body, leading to lower limb pain.